* Reproduce github issue #1047.
* Use RwLock to protect gc_cuttof_lsn
* Eeduce number of updates in test_gc_aggressive
* Change test_prohibit_get_page_at_lsn_for_garbage_collected_pages test
* Change test_prohibit_get_page_at_lsn_for_garbage_collected_pages
* Lock latest_gc_cutoff_lsn in all operations accessing storage to prevent race conditions with GC
* Remove random sleep between wait_for_lsn and get_page_at_lsn
* Initialize latest_gc_cutoff with initdb_lsn and remove separate check that lsn >= initdb_lsn
* Update test_prohibit_branch_creation_on_pre_initdb_lsn test
Co-authored-by: Heikki Linnakangas <heikki@zenith.tech>